The subject invention relates to cleaning and pretreatment of product surfaces for preparing the product surface for a coating application. More specifically, the subject invention relates to cleaning a product surface with a cleaning agent, phosphate, or other conversion coating for paint application.
Pretreatment refers to the treatment of plastic or metallic surfaces such as automobile parts and preparation for subsequent application of a coating to the part. For example, a metallic surface for an automobile body is prepared for subsequent application of electrodeposition paint via an electrodeposition paint tank. Plastic components have also been known to have a surface treatment in a pretreatment system prior to the application of a primer or decorative coating.
A typical metallic vehicle body arrives at a pretreatment apparatus covered with metal forming oils, grinding dust, and other environmental contaminants. These contaminants must be removed to apply a defect-free phosphate coating to prepare the metallic surface to receive corrosion resistance and decorative coatings. Pretreatment includes a series of surface cleaning stages followed by phosphating, or the application of a conversion coating. Zinc phosphate is the principal conversion coating used in the automobile industry today, although iron phosphate is widely used in other applications. The pretreatment process includes a series of steps designed to remove contaminants from the metal surface, convert the surface to an inorganic crystalline coating, and seal the crystalline structure.
One example of an immersion zinc phosphate system includes nine stages: Spray cleaner, immersion cleaner, spray rinse, immersion conditioner rinse, immersion zinc phosphate, immersion rinse, immersion chromic acid rinse, immersion recirculated deionized water rinse, and a spray virgin deionized water rinse. The first two stages, the spray cleaner and immersion cleaner stages, clean the surface of contaminants to prepare the surface to form a tight, adherent, fine grained zinc phosphate coating. Absent a clean surface, the first layer of paint, commonly referred to as an electrodeposition coating, will not adhere properly resulting in paint defects projecting through the top coat or premature corrosion of the metallic surface. The third stage, the spray rinse stage, follows cleaning and rinses the cleaning solution from the metallic surface. Failure to rinse the cleaning solution results in contamination of subsequent chemical treatment stages. The immersion conditioner rinse stage, stage four, increases the nucleation sites on the metallic surface, thereby reducing the amount of zinc phosphate required to coat the surface and improve conversion coating uniformity. The immersion zinc phosphate stage, stage five, applies phosphate crystals to the metal surface, giving the surface corrosion-inhibiting properties and providing an improved base for paint application. Stage six, the immersion rinse, rids the surface of compounds from the zinc phosphate stage that could contaminate stage seven, the immersion chromic rinse stage. The chromic acid rinse removes remaining water soluble compounds from the surface to maximize corrosion protection. While the chromic acid rinse stage is not essential to the phosphating process, corrosion resistance of many substrates has proven to be greatly enhanced by the use of chromic acid. The last two stages, the recirculated deionized water rinse and the spray virgin deionized water rinse, remove all phosphate residue from the surface so as to not contaminate the electrodeposition primer tank.
The first stage uses a series of spray nozzles or other applicators to spray the metal surface and remove contaminants through impingement and chemical cleaning. A typical cleaning chemical includes a caustic base and surfactant content. A tank having, for example, a 30,000 gallon capacity receives run-off from the spray as well as contaminants that are rinsed from the metallic surface. Oils removed from the surface are removed from the tank and transferred to a treatment process. Solution used in the process is recirculated and re-enters the process through the spray nozzles or other applicators.
The contaminants removed from the spray stage collect inside the tank and deplete the cleaning chemicals. Grinding dust and other solid contaminants are known to collect at the bottom of the tank. To clean the tank, the tank is drained and the contaminants are removed from the bottom of the tank. To maintain an optimum chemical content, cleaning occurs approximately every two weeks. Otherwise, the contaminants collect in the tank and deplete the chemical cleaner.
A further problem results when the contaminants collect in the tank close to the spray nozzle inlet that provide the cleaning solution to the spray nozzles. If these contaminants are not allowed to settle to the floor of the tank, they can be drawn into the spray nozzle inlet. These contaminants are known to plug up the inlet, collect in the spray nozzles, and foul the pumps that pump the solution at a desired impingement pressure from the tank to the spray nozzles.
It is therefore an object of the present invention to provide a system that decreases the collection of contaminants inside the first stage proximate to the nozzle inlet resulting in less frequent cleaning of the tank and spray nozzles and increased efficiency of the cleaning stages of the pretreatment process.